What makes a cyclocross bike?

Sorry didn't know where to post this (or even if it is allowed!)
but it is a cycling related question.
At my sons cycling club they practiced for cyclocross today and it looks likes great fun.
I fancy giving it a go and think it would be great for the winter months.
I understand that the brakes are different for greater clearance and likewise the tyres are different.
What I don't know is what else is different between a road bike and a cyclocross bike.
I have a winter bike (Peugeot , it has 9 speed shimano 105).
Is it financially sensible to upgrade the bike. Is it possible to upgrade the bike? If so what might need doing?
As a family we have too many bikes and really don't want another one.
Could I convert my old mountain bike. Would this be easier/cheaper? The bike might be heavier, but I wonder if this is a better route? Must you have drop bars?
Any advice greatly appreciated.

At my sons cycling club they practiced for cyclocross today and it looks likes great fun.
I fancy giving it a go and think it would be great for the winter months.
I understand that the brakes are different for greater clearance and likewise the tyres are different.
What I don't know is what else is different between a road bike and a cyclocross bike.
I have a winter bike (Peugeot , it has 9 speed shimano 105).
Is it financially sensible to upgrade the bike. Is it possible to upgrade the bike? If so what might need doing?
As a family we have too many bikes and really don't want another one.
Could I convert my old mountain bike. Would this be easier/cheaper? The bike might be heavier, but I wonder if this is a better route? Must you have drop bars?
Any advice greatly appreciated.
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Posts
Best thing to do is have a look at one of the (many) manufacturer's websites to compare road and cyclocross bikes. Try Focus, for example. Some of the main differences are:
'crossers have:
cantilever brakes (some have discs), some have a second set of levers mounted on the tops.
wider tyres (typically 32mm).
clearance for mudguards
more "upright" geometry (you may need a smaller frame size).
Drop handle bars are better as you can pull on them to get purchase, and they make it easier to carry the bike over your shoulder.
And of course you need another bike!
Steve
The MTB would also do. Consider that most CX events are tolerant of almost anything for beginners. I have seen plenty of mtb's out there. Just do a bit to help you ie narrower tyres maybe but you could just drag it out of the shed and ride.
When you dig deeper you may find that as well as clearances the geometry of road and CX bikes is different as well as, for some people, the sizing etc. Gears are generally a bit lower all round but often not super low as events are not really hill. £ Peaks excluded of course. An old touring bike maybe a good start if its 531 or similar as it will ave greater clearances. Everything else could be culled from a road bike I suppose.
It is. Get out there as soon as you possibly can and give it a go on your MTB. The season is short and the best thing you could do would be to experience it before it's over. Then you can think about whether you want to do more of it and can decide whether you want a proper 'cross bike for the 2012-13 season.
Ruth